Directional couplers, which are electronic devices that can detect power being transmitted in a particular direction, are used in a wide variety of radio frequency (RF) circuits. For example, a directional coupler may be used in a radar system to detect a reflected wave by separating the indecent wave from the reflected wave, or may be used in a circuit that measures the impedance mismatch of transmission lines. Functionally, a directional coupler has a forward transmission path and a coupled transmission path. The forward transmission path generally has a low loss, while the coupled transmission path couples a fraction of the transmission power that is propagated in a particular direction. There are many different types of coupler architectures that include electromagnetic couples and magnetic couplers. Each of these coupler types may be implemented using different topologies and materials depending on the frequency of operation and the operational environment.
One common application for a directional coupler is the detection of the reflected and transmitted power in a portable radio frequency (RF) device such as a cellular telephone or a portable computing device. The measurement of the transmitted power may be used in a control loop to adjust the output of a power amplifier, while the measurement of the reflected power in conjunction with the measurement of the reflected power may be used to adjust adjustable antenna matching networks. One parameter that limits the accuracy of such reflected power measurements is the directivity of the directional coupler, which is related to the ability of the directional coupler to isolate a measurement from an incident wave to a reflected wave.